Ribonuclease T1 Peptides. I. Synthesis of a Protected N-Terminal Undecapeptide Containing Asparagine Residue in Third Position

A protected undecapeptide containing the possible N-terminal sequence of ribonuclease T1, namely benzyloxycarbonyl-l-alanyl-S-benzyl-l-cysteinyl-l-asparaginyl-l-tyrosyl-l-threonyl-S-benzyl-l-cysteinyl-glycyl-l-seryl-l-asparaginyl-S-benzyl-l-cysteinyl-l-tyrosine ethyl ester (XXI), was synthesized by coupling of benzyloxycarbonyl-alanyl-S-benzylcysteinyl-asparaginyl-tyrosine azide with heptapeptide ester, threonyl-S-benzylcysteinyl-glycyl-seryl-asparaginyl-S-benzylcysteinyl-tyrosine ester, which is derived from p-methoxybenzyloxycarbonyl-heptapeptide ester by the action of trifluoroacetic acid. The optical homogeneity of the protected undecapeptide (XXI) was established by deblocking of XXI and subsequently digesting the resulting free undecapeptide with leucine aminopeptidase.